Plains in the southeastern Elysium region of Mars are identified as Upper Amazonian (200-500 Ma) resulting from flood eruptions of low-viscosity lavas. About (10^{5} \mathbf{k m}^{3}) of lava were erupted and would have released large quantities of volatiles into the Martian environment. Assuming that the lavas contained (1.0 \mathrm{wt} % \mathrm{H}{2} \mathrm{O}), about (10^{16} \mathrm{~kg}) of water would have been released, corresponding to an increase of at least a factor of (10^{3}) times the present atmospheric water mass. Total (\mathrm{CO}{2}) release would have been about (10^{16} \mathrm{~kg}), comparable to the present atmospheric (\mathrm{CO}{2}) mass. The amounts of other volatiles are (10^{15} \mathrm{~kg} \mathrm{~S} ; 10^{13} \mathrm{~kg} \mathrm{Cl}), and (10^{13} \mathrm{~kg} \mathrm{~F}). Potential effects of these volatiles include: (1) initial surface warming due to an (\mathrm{SO}{2}) greenhouse effect; (2) possible subsequent surface cooling due to sulfate aerosol formation; (3) formation of duricrust due to the deposition of mobilized sulfates and chlorides on the surface; (4) local flooding and channeling due to precipitation; and (5) development of the perennial polar caps. O 1993 Academic Press, Inc.